Apparently, adjustable paragraph or line spacing is possible only in the Windows desktop version of OneNote. It doesn't seem possible in OneNote for Mac, MX for Windows 8/8.1/10, Online, iOS, or Android. It may be possible in OneNote for Windows 10 for phones, but I highly doubt it.

Whenever I copy text from the web into OneNote Online, the text has somewhat larger paragraph spacing but not line spacing.